Key:
White Units = Normal
Blue Units = Magic
Yellow Units = Rare
Orange Units = Epic
Red Units = Unique
Pink Units = Hell
Green Units = Hidden (Not listed in-game)

Tier 1 (Cheap Units):
Marine + Marine = Warmonger
Stalker + Stalker = Dark Stalker
Roach + Roach = Roach-Primal Strain
Vulture + Vulture = Diamondback
Hydralisk + Hydralisk + Hydralisk = Hydralisk-Primal Strain x 2
Archon + Archon + Archon = Immortal x 2
Sniper + Sniper = Big Sniper
High Templar + High Templar = Karas
Lurker + Lurker = Lurker
Goliath + Goliath = Spartan Company
Siege Tank + Siege Tank + Siege Tank = Siege Breaker x 2
Marauder + Marauder + Marauder = Hammer Securities x 2

Tier 2 (High Class):
Hammer Securities + Warmonger = Death Head
Dark Stalker + Immortal = Stone Zealot
Roach-Primal Strain + Hydralisk-Primal Strain = Ravasaur
Diamondback + Siege Breaker = A.R.E.S
Big Sniper + Hammer Securities = Spectre
Karas + Immortal = Tassadar
Lurker + Hydralisk-Primal Strain = Brutalisk
Spartan Company + Siege Breaker = Blackhammer

Tier 3 (Masterpiece):
Death Head + Spectre = Jim Raynor
Stone Zealot + Tassadar = Zeratul
Ravasaur + Brutalisk = Kerrigan
A.R.E.S + Blackhammer = Arch Angel

Tier 4 (Epic):
Kerrigan + Zeratul = Hybrid Reaver
Jim Raynor + Zeratul = Hybrid Dominator
Arch Angel + Arch Angel = Odin
Kerrigan + Jim Raynor = Alexei Stukov

Tier 5 (Unique):
Hybrid Reaver + Hybrid Reaver + Hybrid Dominator = Leviathan
Odin + Odin + Alexei Stukov = Hyperion
Hybrid Dominator + Hybrid Dominator + Odin = Void Seeker
Alexei Stukov + Alexei Stukov + Hybrid Reaver = Nova

Hidden Units:
BIONIC:
Tauren Spacemarine = Warmonger + 2x Maruder
Tauren Toilet = 2x Tauren Spacemarine + Dark Immortal
Tosh= 2x Spectre+ 3x Hammer Security
Egon Statman = 8x Infested Terrans (spawed from Alexei Stukov) + 2x Death Head
Tychus = 3x Jim Raynor + 2x Alexei Stukov
Dominion Marauder = 4x Hammer Security (At rank SSS or higher) + 8x Infested Terran (Spawed from Alexei Stukov)
Commando Raynor = 6x Jim Raynor + Nova

ZERG:
Swarmhost = Lurker + 2x Hydralisk
Viper = 4x Locust (spawned from Swarmhost) + 4x Hydralisk
Drone = 2x Hydralisk burrowed + Roach burrowed
Vile Roach = 3x Roach burrowed + 2x Dark Stalker
Flying Queen = 2x Swarmhost burrowed + Ravasaur
Spore Cannon (Middle turret) = 12x Drone (At rank S). You have to place the colonies like this in the fusion station (0 = Empty X = Colony): 0XX0 - XXXX - XXXX - 0XX0
Kraith = Hybrid Reaver (At rank X or higher) + 4x Brutalisk
K5 Kerrigan = Leviathan + Hybrid Reaver + 2x Kerrigan

PROTOSS:
Dark Immortal = 4x Immortal
Sentry = Karas + Dark Stalker + 2x Archon
Colossus = Hybrid Dominator + 4x Stonezealot
Tempest= 2x Tassadar + 3x Immortal
Warpprisma = Sentry + 2x hightempler(when they Fusion to Archon)
Voidray = Warpprisma + Hybrid Dominator
Colossus = Hybrid Dominator + 4x Stone Zealot
Mothership = 3x Zeratul + 2x Hybrid Dominator
Selendis = Mothership (At rank X or higher and +3 Gas Upgrade) + Void Seeker (At rank X or higher and +3 Gas Upgrade) + Sentry (You need sentry to give them the attack speed buff at the fusion station)

MECHANIC:
Widowmine = 12 Spider mine (Spawned from Voltures)
Warhound = 2x Sparta Company + Siegetank (Siegemode)
Draken Lazer drill= 2x A.R.E.S + 3x Siege Breaker
Gorgoncruiser = Hyperion + 3x Alexei Stukov
Raven = 2x Warhound + 2x Widowmine + Arch Angel
Garbage = 4x Sparta Company + 4x Hammer Security
HaNwinnerS Nuke System = Garbage + Tauren toilet
Arch Angel Fighter Mode = 4x Arch Angel (At rank SSS or higher, Merge Mode needs to be = OFF)
Gorgoncruiser = Hyperion + 3x Alexei Stukov